This invention relates to the surface treatment of organic pigments used in waterborne coatings with certain phosphoric acid monoesters.
Traditional methods for the surface treatment of organic pigments were designed for solvent borne coatings. The surfaces of both untreated and conventionally treated organic pigments tend to be hydrophobic. However, because of the trend away from solvent-based coatings toward waterborne coatings, surface treatments that allow the organic pigment to be more readily dispersed and stabilized in waterborne systems would be desirable. That is, a more hydrophilic pigment surface is desired.
Pigment dispersions containing phosphoric acid esters (also referred to as phosphate esters) have been described. However, surface treatment of organic pigments with such phosphoric acid esters has generally not been described. For example, U.S. Pat. No. 4,872,916 discloses aqueous pigment dispersions in which the dispersing agent is an ethoxylated alkylphenol phosphate diester. However, in addition to disclosing phosphate diesters rather than monoesters, this patent is directed to preparation of aqueous dispersions containing relatively large quantities of phosphate esters as dispersing aids rather than surface treatment of pigments. U.S. Pat. No. 4,891,401 discloses polymeric pigment dispersants based on polymers having phosphorus-containing acrylate units. However, this patent does not disclose dispersing agents based on phosphoric acid monoesters such as used in the present invention and is directed to aqueous dispersions rather than surface-treatment. U.S. Pat. No. 4,927,463 discloses aqueous gypsum dispersions containing polyoxyalkylene or (alkylphenoxy)alkyl phosphate monoesters and diesters for use in coating and filling paper and cardboard but does not disclose organic pigments or surface treatment of pigments. U.S. Pat. No. 5,073,585 discloses aqueous coatings containing calcium or magnesium pigments and/or fillers, aqueous secondary dispersions of a (meth)acrylate copolymer, and alkyl- and phenyl-substituted oxyalkyl monoesters of phosphoric acid but does not disclose organic pigments or surface treatment of pigments. British Patent 2,090,876 discloses aqueous dispersions of water-insoluble pigments, dyes, and optical brighteners and water-soluble crypto-ionic dispersing agents such as polyoxyalkylene aryl phosphate esters but also does not mention surface treatment. German Offenlegungsschrift 2,414,455 discloses pigment dispersions containing certain sulfonates and oxyethylated fatty amines and optional long-chain dialkyl phosphates that are structurally different from the phosphoric acid monoesters of the present invention. Moreover, the German application does not disclose surface treatment. Polish Patent 127,761 discloses pigment pastes containing inter alia (i) a mixture of ethoxylated alkylphenols and polyethylene glycol ether and (ii) a phosphate glaze as a water-softening agent. This patent does not appear to disclose phosphate monoesters such as used in the present invention.
Although certain phosphoric acid esters have been described as useful for surface treatment of pigments, phosphate monoesters such as those used in the present invention have not been described. For example, U.S. Pat. No. 4,323,396 discloses organic pigments treated with polysaccharides containing ionic groups, including an anionic phosphate ester based on potato starch, but does not disclose phosphate monoesters such as used for the present invention. Australian Patent 499,672 discloses a comminution process in which pigments are ground in water containing a fugitive auxiliary, such as a phosphate ester of long chain alcohols or alcohol alkoxylates, after which the fugitive auxiliary must be hydrolytically deactivated by steam distillation or high-temperature drying to obtain pigments having improved tinctorial properties.
It has now been found that the adsorption of certain phosphoric acid monoesters according to the present invention onto pigment surfaces increases the hydrophilicity of the pigment. Consequently, surface modification with such phosphoric acid monoesters provides organic pigments having improved dispersibility in waterborne systems, improved color development, and lower pigment paste viscosity.